driver core: Avoid NULL pointer dereferences in device_is_bound()

If device_is_bound() is called on a device that's not been registered
yet, it will attepmt to dereference dev->p which is NULL, so avoid
that by checking dev->p in there against NULL.

Fixes: 6b9cb427

 "device core: add device_is_bound()"
Reported-and-tested-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>